Voltage source inverters are utilized in AC motor drive, utility interface, and an uninterruptible power supply (“UPS”) applications as a means for converting DC to AC electrical power. A traditional voltage source inverter generates a low frequency output voltage with controllable magnitude and frequency by programming high-frequency voltage pulses. The high frequency voltage pulses open and close switches to expose a load to pulses of DC current. An inverter of this type is said to be using pulse width modulation (“PWM”). Timing, duration, and voltage of the pulses simulate the peaks and troughs of traditional sinusoidal alternating current. Where the load has an inherent inductive nature, such as windings of a motor, the pulses approximate the sinusoid without significant high frequency harmonics.
To handle larger and larger input voltages, larger switching transformers are needed. Where silicon fabrication has not kept up with the need for greater power, a multi-level inverter topology has arisen. The topology equally divides two input voltage sources, thereby allowing twice the total voltage at the output for the same capacity transistor. The inverter was further refined for applications that do not have divided input voltage sources to have instead a series connected capacitor bank defining a neutral point-clamped multi-level voltage source inverter.
The three-level voltage source inverter is one of the most popular topologies for three-phase three-level voltage source inversion. The advantages of the three-level voltage source inverter are:                1) Because of the redundancy of the switches, voltage across any one switch is only half of the DC bus voltage;        2) Switching losses are cut in half due to the reduced harmonics present in the output wave forms for the same switching frequency; and        3) The power rating increases.        
The literature recognizes certain drawbacks, as well, in the three-level voltage source inverter. Such inverters require complex control circuitry, each of the redundant switches add to the price of the voltage source inverter, and the charge at the mid point between the two DC linking capacitors can accumulate when switching is not balanced.
Due to improvements of fast-switching power semiconductor devices and machine control algorithms, high performance PWM dc-ac inverters find a growing interest. Among requirements of PWM inverters, full utilization of the dc bus voltage is extremely important to achieve maximum output torque under all operating conditions for synchronous motor drive applications. With a voltage source inverter, the output voltage is bounded by dc link voltage in the form of hexagon. When a reference voltage vector exceeds the hexagon boundary, the reference voltage cannot be applied to the motor. This state of exceeding the hexagon boundary is referred to as ‘overmodulation’. When an inverter operates in continuous overmodulation mode at high speed, inverter output voltage contains substantial sub-carrier frequency harmonics. As a result, drive performance degrades considerably.
Though fundamental component voltage gain and current waveform characteristics in the overmodulation region are well improved, these schemes are suitable only for an open loop (volts-per-hertz) controlled synchronous motor drive, not a vector-controlled synchronous motor drive.
Vector controlled permanent magnet synchronous motor drive requires closed loop current control with fast dynamic characteristics. Although the performance of current PWM inverters meets the requirement within the voltage boundary, in the overmodulation region the drive performance significantly degrades and the bandwidth of the regulator is shrunk. Therefore, the designed performance of current PWM inverters is guaranteed only in the linear modulation region. When inverter enters continued overmodulation region, the rotor flux should be weakened to reduce induced back-electromotive force voltage. It is, however, difficult to weaken rotor flux sufficiently in the transient state such as reference change, load disturbance, and sudden drop of utility voltage. This is because this transient state is much shorter than the rotor time constant in general. Therefore, in this case, a proper dynamic overmodulation scheme should be implemented because the modulation scheme determines the dynamic performance of the motor drive.
There is then an unmet need in the art for a proper dynamic overmodulation scheme for a three-level inverter.